When President Barack Obama decides to give a keynote interview to Fox News it is a sure sign that something is stirring.

For the past eight years, the president has mostly given a wide berth to the Right-wing cable channel that has been a stern critic of his administration and its policies - apart from two interviews, one of which took place during the Super Bowl.

Yet this weekend, Mr Obama will appear on Fox News Sunday with Chris Wallace in the middle of the rowdiest primary campaign in living memory in what can only be an eloquent indication of intent.

The last time he was interviewed by Wallace on the high profile Sunday show - one of American television’s political agenda-setters - was when the-then Illinois senator was running for president in 2008.

Barack Obama will go on Fox News to counter the rise of Donald Trump.Credit:
Getty

In drastically changing the habits of a presidency, Mr Obama is making it clear that he is back on the campaign trail.

But the president, who leaves the White House in January, is not campaigning for office - a third presidential term being constitutionally barred - but primarily against the election of others, notably Donald Trump.

Mr Obama has in recent weeks become increasingly vocal in his denunciations of Mr Trump’s reckless policy pronouncement and inflammatory rhetoric.

Mr Trump’s dismissal of the Nato alliance as obsolete and his suggestion that Japan and South Korea could be allowed have nuclear weapons to defend themselves against North Korea has given the president the chance to deploy the gravitas of his office to shoot down the provocative Republican front-runner.

This week he said unambiguously that Mr Trump’s ideas were hurting America’s relations with other countries.

"I am getting questions constantly from foreign leaders about some of the wackier suggestions that are being made,” he said. “They don't expect half-baked notions coming out of the White House. We can't afford that."

"I am getting questions constantly from foreign leaders about some of the wackier suggestions that are being made"

Barack Obama

On the South Korea and Japan proposals, the president told a nuclear security summit in Washington last week that they “tell us that the person who made the statements doesn't know much about foreign policy, or nuclear policy, or the Korean Peninsula..…Or the world generally.”

The president’s goal, his allies have said, is to mobilise the coalition that won him the 2008 and 2012 elections to ensure a Democratic victory in November - thereby protecting his presidential legacy, which would be threatened if either Mr Trump or his deeply conservative Republican rival Ted Cruz took the White House.

By confronting Mr Trump directly, some may charge that he risks lowering himself to the brash billionaire’s level and making it appear personal. The two do, after all, have a history. Mr Trump was in the vanguard of the doomed “birther” movement that sought to prove Mr Obama was not American-born, a charge he was forced to rebut by releasing his birth certificate.

Yet there are signs that Mr Trump’s bizarre and often troubling antics have helped Mr Obama’s standing and boosted his popularity by making him appear presidential in comparison - even in the eyes of at least some Republican voters.

Now might be the perfect time to out-Fox Mr Trump on the campaign trail - and persuade sceptical Right-wing cable viewers that there are things about his legacy worth preserving.